def _main():
    input_str = input("请输入学生的各课成绩，用英文逗号（,）分隔：")
    split = input_str.split(",")
    length = len(split)
    try:
        marks_per = [(float(s) / length) for s in split]
    except ValueError:
        print("输入格式不正确。请检查输入。")
        return
    import math
    print("平均成绩为：%.2f" % math.fsum(marks_per))


if __name__ == "__main__":
    _main()
